Reclassifications Out of Accumulated Other Comprehensive Income - Reclassifications Out of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net gains on sales of securities									$ 61	$ 302	$ 47
Provision for income taxes	$ (526)	$ (435)	$ (267)	$ 118	$ (309)	$ (444)	$ (314)	$ (501)	(1,110)	(1,568)	(10,958)
Net income	$ 10,732	$ 10,084	$ 9,466	$ 9,418	$ 9,925	$ 9,581	$ 8,998	$ 7,709	39,699	36,213	22,301
Salaries and employee benefits									(44,014)	(42,710)	$ (40,517)
Reclassification Out of Accumulated Other Comprehensive Income [Member] | Unrealized Gains and Losses on Available-for-Sale Securities [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net gains on sales of securities									61	302
Provision for income taxes									(17)	(85)
Net income									44	217
Reclassification Out of Accumulated Other Comprehensive Income [Member] | Accretion of Unrealized Losses Transferred [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net gains on sales of securities									(1,022)	(1,477)
Provision for income taxes									269	391
Net income									(753)	(1,086)
Reclassification Out of Accumulated Other Comprehensive Income [Member] | Prior-Service Costs [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Salaries and employee benefits									(114)	(14)
Reclassification Out of Accumulated Other Comprehensive Income [Member] | Actuarial Gains (Losses) [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Salaries and employee benefits									(1,351)	(1,610)
Reclassification Out of Accumulated Other Comprehensive Income [Member] | Amortization of Defined Benefit Pension Items [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income before taxes									(1,465)	(1,624)
Provision for income taxes									412	457
Net income									$ (1,053)	$ (1,167)